Fast food is a type of mass-produced food designed for commercial resale, with a strong priority placed on speed of service. It is a commercial term, limited to food sold in a restaurant or store with frozen, preheated or precooked ingredients and served in packaging for take-out/take-away. Fast food was created as a commercial strategy to accommodate large numbers of busy commuters, travelers and wage workers. In 2018, the fast food industry was worth an estimated $570 billion globally.

The fastest form of "fast food" consists of pre-cooked meals which reduce waiting periods to mere seconds. Other fast food outlets, primarily hamburger outlets such as McDonald's, use mass-produced, pre-prepared ingredients (bagged buns and condiments, frozen beef patties, vegetables which are prewashed, pre-sliced, or both; etc.) and cook the meat and french fries fresh, before assembling "to order".

Fast food restaurants are traditionally distinguished by the drive-through. Outlets may be stands or kiosks, which may provide no shelter or seating, or fast food restaurants (also known as quick service restaurants). Franchise operations that are part of restaurant chains have standardized foodstuffs shipped to each restaurant from central locations.

Many fast foods tend to be high in saturated fat, sugar, salt and calories. Fast food has been linked to increased risk of cardiovascular disease, colorectal cancer, obesity, high cholesterol, insulin resistance conditions and depression. These correlations remain strong even when controlling for confounding lifestyle variables, suggesting a strong association between fast food consumption and increased risk of disease and early mortality.